Preserve leading slashes in request path_url

src/requests/adapters.py

@@ -545,8 +545,6 @@ def request_url(self, request, proxies):
             using_socks_proxy = proxy_scheme.startswith("socks")
 
         url = request.path_url
-        if url.startswith("//"):  # Don't confuse urllib3
-            url = f"/{url.lstrip('/')}"
 
         if is_proxied_http_request and not using_socks_proxy:
             url = urldefragauth(request.url)

tests/test_adapters.py

@@ -1,8 +1,14 @@
 import requests.adapters
 
 
-def test_request_url_trims_leading_path_separators():
-    """See also https://github.com/psf/requests/issues/6643."""
+def test_request_url_preserves_leading_path_separators():
+    """See also https://github.com/psf/requests/issues/6711.
+
+    S3 presigned URLs with keys starting with '/' produce paths like
+    '//key_name'. We should preserve leading slashes to avoid breaking signatures.
+    """
     a = requests.adapters.HTTPAdapter()
-    p = requests.Request(method="GET", url="http://127.0.0.1:10000//v:h").prepare()
-    assert "/v:h" == a.request_url(p, {})
+    p = requests.Request(
+        method="GET", url="https://bucket.s3.amazonaws.com//key_with_leading_slash.txt"
+    ).prepare()
+    assert "//key_with_leading_slash.txt" == a.request_url(p, {})